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Alkyl Halides

Alkyl Halides

Structural Properties
Alkyl halides are halogen-substituted alkanes wherein one or more hydrogen atoms of an alkane is replaced by a halogen atom such as fluorine, chlorine, bromine, or iodine. The carbon atom in an alkyl halide is bonded to the halogen atom, which is sp3-hybridized and exhibits a tetrahedral shape.
Unlike alkyl halides, compounds in which a halogen atom is bonded to an sp2 -hybridized carbon atom of a carbon-carbon double bond (C=C) are called vinyl halides. Whereas aryl...

Reactions at the Benzylic Position: Halogenation

Reactions at the Benzylic Position: Halogenation

Benzylic halogenation takes place under conditions that favor radical reactions such as heat, light, or a free radical initiator like peroxide.

Radical Substitution: Allylic Bromination

Radical Substitution: Allylic Bromination

In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is formed when propene is treated with bromine at room temperature. In contrast, propene undergoes allylic substitution in non-polar solvents at high temperatures to give 3-bromopropene. In order to avoid the addition reaction, the bromine concentration must be kept as low as possible throughout the reaction. This can be achieved using N-bromosuccinimide...

Halogenation of Alkenes

Halogenation of Alkenes

Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.

Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...

Radical Halogenation: Thermodynamics

Radical Halogenation: Thermodynamics

The thermodynamic favorability of a reaction is determined by the change in Gibbs free energy (ΔG). ΔG has two components- enthalpy (ΔH) and entropy (ΔS). The entropy component is negligible for alkane halogenation because the number of reactants and product molecules are equal. In this case, the ΔG is governed only by the enthalpy component. The most crucial factor that determines ΔH is the strength of the bonds. ΔH can be determined by comparing the energy between bonds broken and bonds...

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Related Experiment Video

Updated: May 19, 2026

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

Published on: July 5, 2019

Adsorbate alignment in surface halogenation: standing up is better than lying down.

Kai Huang¹, Iain R McNab, John C Polanyi

¹Lash Miller Chemical Laboratories, Department of Chemistry and Institute of Optical Sciences, University of Toronto, Toronto, Ontario, Canada.

Angewandte Chemie (International Ed. in English)

|August 7, 2012

Summary

Vertical alignment of bromoalkanes on silicon surfaces enhances reactivity for bromine atom transfer. This study reveals orientation-dependent chemical reactions on silicon using scanning tunneling microscopy.

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Area of Science:

Surface Science
Chemical Physics
Materials Chemistry

Background:

Understanding adsorbate-molecule interactions is crucial for surface chemistry.
The reactivity of halogenated alkanes on semiconductor surfaces is of significant interest.
Controlling molecular orientation influences surface reactions.

Purpose of the Study:

To investigate the effect of adsorbate alignment on bromine atom transfer to a silicon surface.
To compare the reactivity of vertically versus horizontally aligned bromoalkanes.
To explore orientation-dependent reactivity in both thermal and electron-induced reactions.

Main Methods:

Scanning Tunneling Microscopy (STM) was employed to study the reactions.
Experiments were conducted on a silicon(111)-7×7 surface.
1-bromopropane and 1-bromopentane were used as adsorbate molecules.

Main Results:

The vertical alignment of 1-bromopropane and 1-bromopentane was found to be more reactive.
This enhanced reactivity was observed in both thermal and electron-induced bromination reactions.
Molecular orientation significantly impacts the efficiency of bromine atom transfer.

Conclusions:

Adsorbate alignment is a critical factor controlling the reactivity of bromoalkanes on silicon surfaces.
Vertical orientation facilitates more efficient bromine atom transfer.
These findings provide insights into designing surface reactions with controlled molecular orientation.